The invention relates to the generation of television blanking information and more particularly, to the digital generation of PAL blanking region information via a dynamic offset circuit which makes the information appear orthogonal after digital to analog conversion.
In a television studio, or when otherwise recovering digitally sampled video signals from a recording media or other noisy source such as a satellite receiver, it is necessary to re-insert new video blanking, sync and burst timing information. That is, in such video processes, a sync generator is used to provide video sync blanking and burst signals, in order to maintain the proper relationship of all synchronizing information relative to the active video signal. In a system employing the NTSC color television standard, it is relatively simple to maintain the phase relationship between the color subcarrier and the horizontal sync of the television signal because there is a direct relationship between the two signals. That is, one signal is generated directly from the other whereby a fixed phase relationship between the signals is readily reproduced.
In the PAL standard, however, the relationship between the horizontal frequency and the color subcarrier frequency is more complex as shown by the relationship F.sub.sc =1135/4 F.sub.h +25, where F.sub.sc is the color subcarrier and F.sub.h the horizontal frequency. This relationship results from the 25 Hz offset which is used in the PAL standard.
Stated more simply, in a standard rectangular television picture the horizontal blanking information such as sync and burst are orthogonally related to a vertical line along the left hand side of the picture. In an NTSC color television standard, since there is the fixed frequency relationship between horizontal frequency and the color subcarrier frequency, an orthogonal blanking region configuration readily is achieved. That is, the timing of all blanking region information begins exactly on the vertical line, regardless of whether the video signals are being processed in the analog or digital domain.
Likewise, in the PAL standard, if the video signals are being processed in the analog domain an orthogonal blanking region configuration also readily is achieved. That is, since an analog signal is not sampled and inherently is a continuous signal, the sync blanking and burst edges are readily generated in an orthogonal configuration. However, problems arise when a PAL-encoded video signal is processed entirely in the digital domain, as further discussed below.
Presently available time base correctors, (TBC's) digital video tape recorders (VTR's), and the like, typically process various portions of the video signal by analog means, particularly in the processing amplifier and D/A converter area. In such schemes, the video signal is put through a path which includes various complex digital processes culminating in digital-to-analog (D/A) conversion. The various timing signals however, are processed in a separate channel and are put through other analog processes unrelated to the digital video signal processes. Thus, when the video signal and the timing signals are recombined as required prior to D/A conversion, there are inherent instabilities in the timing between the blanking region information and the active video signal caused by drift, noise, etc.
However, notwithstanding the problem of instability, it is highly desirable in this generation of VTR's and associated TBC's that the video signal be processed entirely in the digital domain. Optimum video signal processing is achieved in the digital domain since the television picture is defined very accurately by the digital samples, and analog associated problems such as instability and signal drift inherently are overcome.
As previously discussed, in a digital PAL system the color subcarrier and thus the sampling clock are offset from the horizontal scanning frequency by the frame scanning frequency of 25 Hz. Accordingly, when blanking region information is re-inserted, the samples cannot be taken along the vertical line of previous mention. As a result the blanking interval information is non-orthogonal relative to the rectangular television picture. It follows that the 25 Hz offset in a digital PAL system causes intolerable horizontally displaced steps in the blanking interval timing signals, which cause the generation of an undesirable family of blanking, sync and burst envelopes that do not represent the instantaneous timing of the original television signal.
The present invention overcomes the disadvantages of processing video signals in the analog domain, while overcoming the problems of non-orthogonality of the blanking region information caused by the 25 Hz offset in a digital PAL-encoded color television system. The video signal and the timing information may be processed entirely in the digital domain, which is a decided advantage, for example, in a time base corrector, a digital VTR, etc. The invention digitally generates the blanking region information via a non-orthogonal circuit while processing the information with the same clock that processes the video data. To this end, a dynamic offset circuit is provided which makes the blanking region information appear orthogonal when the subsequent process of digital-to-analog conversion is performed, whereby the blanking interval timing signals of successive television frames or pictures are precisely synchronized.
More particularly, the envelopes of the blanking interval signals are stored as gain points or numbers in digital format in a programmable read-only-memory (PROM). A plurality of waveforms describing the desired envelope are stored, each with a slightly different phase value and in sufficient number to describe one sampling clock cycle. When processing a video signal, the gain points representing the waveforms are sequentially addressed at a 25 Hz rate, whereby the resulting output blanking interval information is offset by 25 Hz to correct for the PAL 25 Hz offset. Thus the blanking interval information is assembled orthogonally to the television scanning frequency.
To this end, a binary counter generates a binary word of, for example, 7-bits, representing the instantaneous phase of the 25 Hz waveform. The four least significant bits (LSB's) are used to address the PROM of previous mention, which contains gain numbers corresponding to sixteen phased envelope waveforms describing one quadrant of a color subcarrier cycle (Fsc). The two most significant bits (MSB's) from the counter represent the four quadrants of the full Fsc cycle and are used to control the phase of the start time of successive quadrants of the cycle. The start time actually is controlled by a presettable binary counter that is clocked at a 4 times subcarrier rate. It is configured as a shift register and coupled to receive the two MSB's from the binary counter.